Dimeric Palladium Research Articles

Thiocyanate Functionalised Ionic Liquids: Synthesis, Characterisation and Reactivity

AbstractThe reaction of chloromethyl thiocyanate with 1‐methylimidazole affords the imidazolium salt, 1‐thiocyanomethyl‐3‐methylimidazolium chloride, [C1SCNmim]Cl (1a). Exchange of the chloride anion in 1a by [PF6]–, [BF4]–, [Tf2N]– or [AuCl4]– affords salts 1b, 1c, 1d and 1e, respectively. Salts [C1SCNmim][BF4] (1c) and [C1SCNmim][Tf2N] (1d) melt below 100 °C and can therefore be classified as ionic liquids. The reaction of 1a with PdCl2 affords [C1SCNmim]2[PdCl4] (2a), whereas 1c reacts with PdCl2 to form the complex [PdCl2{C1SCNmim}2][BF4]2 (2c). In the presence of HNO3 both 1a and 1c react with PdCl2 with the loss of HCN to form a sulfide‐bridged palladium dimer [PdCl2{C1Smim}]2 (3). The structures of 1a, 1e, 2a and 3 have been determined in the solid state by single‐crystal X‐ray diffraction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis and Computational Studies of Palladium(I) Dimers Pd2X2(PtBu2Ph)2(X = Br, I): Phenyl versus Halide Bridging Modes

The synthesis and characterization of the new palladium(I) dimer Pd2Br2(μ-PtBu2Ph)2 (8) is reported herein. The single-crystal X-ray analysis of this compound has shown that the phosphine acts as a bridging ligand between the two palladium centers, using the phosphorus atom as well as the ipso- and ortho-carbon atoms of the phenyl substituents. An analysis of the electronic structure indicates that the bond between the remote palladium center and the bridging ligand is stabilized by two distinct electronic components, one with the CC bond of the arene, the other an agostic-type interaction with the P−C bond. The steric bulk at the phosphorus center causes distinct changes in the relative contributions of these two components and, hence, distorts the structure. In contrast, the iodide analogue (9) adopts a completely different structure, where the phosphines are terminally coordinated and the halide acts as a bridging ligand.

Preparation of a palladium dimer with a cobalt-containing bulky phosphine ligand: Its application in palladium catalyzed Suzuki reactions

A palladium dimer with a cobalt-containing phosphine ligand, {(μ-PPh 2CH 2PPh 2)Co 2(CO) 4(μ,η-( tBu) 2PC CC 6H 4-κC 1)Pd(μ-Cl)} 2 ( 3), was prepared from the reaction of its monomer precursor, (μ-PPh 2CH 2PPh 2)Co 2(CO) 4(μ,η-( tBu) 2PC CC 6H 4-κC 1)Pd(μ-OAc) ( 2), with LiCl. The crystal structure of 3, determined by X-ray diffraction methods, revealed a doubly chloride-bridged palladium dimeric conformation. Suzuki coupling reactions of bromobenzene with phenylboronic acid were carried out catalytically using these two novel palladium complexes 2 and 3 as catalyst precursors. Factors such as the molar ratio of substrate/catalyst, reaction temperature, base and solvent that might affect the catalytic efficiencies were investigated. As a general rule, the performance is much better by employing 3 than 2 as the catalyst precursor.

Synthesis and structural investigations of bulky imino- and amido-phosphine palladium dimers

The reactions of palladium(II) dimers [Pd 2Br 2(L1) 2] and [Pd 2Br 2(L2) 2] (where L1 − is [Ph 2PCH C(Ph)N(2,6- i Pr 2C 6H 3)] − and L2 − is [Ph 2PCH C(Ph)N(2,6-Me 2C 6H 3)] −) with AgBF 4 in a mixture of CH 2Cl 2 and MeOH give palladium(I) dimers [Pd 2(HL1) 2][BF 4] 2 and [Pd 2(HL2) 2][BF 4] 2, respectively. These exhibit unusual coordination geometries of the metal centre. Density functional theory (DFT) calculations showed that a phenyl ring of the bridging phosphine is involved in bonding via a delocalised metal-phosphine–phenyl interaction. The remarkable kinetic stability of these palladium(I) species may explain the early termination steps in the CO/ethylene copolymerisation reaction catalysed by Pd(II)–amidophosphines or enolisable Pd(II)–iminophosphines.

Synthesis and characterisation of benzyl phosphino-thioether and -thiolato Pd(ii) complexes and their applications in Suzuki coupling reactions

The synthesis of the benzyl phosphinothioether derivatives Ph(2)PCH(2)CH(Et)SR and their corresponding palladium complexes are reported, where R = CH(2)Ph, R = CH(2)-3,5-Me-C(6)H(3) and R = 1-CH(2)C(10)H(7)(5). Crystallographic data obtained for the complexes Pd(3)Cl(2) and Pd(4)Cl(2) show intra- and inter-molecular pi-pi interactions between the aromatic rings on the P and S substituents, and NOE experiments for Pd(4)Cl(2) show that these interactions persist in solution. The performance of the phosphinothioether palladium complexes in aryl-aryl cross-coupling reactions is compared with that of the corresponding complex of the parent phosphinothiolato ligand Ph(2)PCH(2)CH(Et)S(-)(1). High turnover numbers up to 2000000 are reported for the coupling of bromobenzene, using the palladium dimer [Pd(1)I](2) as the catalyst precursor. Kinetic studies show a linear dependence of the reaction on catalyst loading. The effect of other variables on the cross-coupling reaction, such as temperature, solvent and base, is also reported.

Synthesis, Structural, and Electron Topographical Analyses of a Dialkylbiaryl Phosphine/Arene-Ligated Palladium(I) Dimer: Enhanced Reactivity in Suzuki−Miyaura Coupling Reactions

The treatment of bis(2-(dicyclohexylphosphino)-2',6'-dimethoxybiphenyl)PdCl2 with AgBF4 produces an air-stable phosphine/arene-ligated Pd(I) dimer with two seemingly identical Pd-arene interactions by X-ray crystallography. However, NMR and theoretical electron topographical analyses of this complex distinguish between these two interactions. One interaction is classified as an arenium-like complex, while the other is classified as a pi-interaction. Additionally, this complex is a suitable precatalyst for high yielding Suzuki-Miyaura coupling reactions in short reaction times.

Allyl(Acetylacetonato)Palladium (II) Complexes: Versatile Precursors for the Synthesis of Dimeric Allylpalladium (II) Complexes

The facile synthesis and characterisation of several new dimeric (η3-allyl)palladium (II) complexes of the type, [Pd(μ-L)2(η3-C3H4R)2] [R = H or Me; LH = 4-CH3C6H4SH; 3,5-R'2pzH (R’ = But or Ph]; ArNNNHAr (Ar = 4-FC6H4); PhNC(Ph)NHPh] are reported.

Conversion of chlorophenols into cyclohexane by a recyclable Pd-Rh catalyst

Mono and polychlorinated phenols that undergo only partial detoxification by conventional hydrogenation methods undergo exhaustive hydrogenation in the presence of a combined silica sol–gel entrapped catalyst composed of metallic palladium and chloro(1,5-cyclooctadiene)rhodium dimer. While the mono-, di-, tri- and tetrachlorophenols are converted into cyclohexane in a variety of solvents at 120 °C under 27.6 bar H 2, the pentachlorophenol requires the presence of toluene. Experiments conducted under milder conditions revealed that the hydrodechlorination of the chlorophenols proceeds via gradual evolvement of the chlorine atoms forming HCl and phenol. The C C bonds of the latter compound are saturated stepwise. The 1-cyclohexenol accumulates as cyclohexanone and is further hydrogenated to cyclohexanol. HCl-catalyzed dehydration yields cyclohexene which is readily transformed to cyclohexane. The initial hydrodechlorination of 2-chlorophenol follows a first order rate law with rate constant k = 5.0 × 10 −4 s −1 at 100 °C. The ceramic combined catalyst is leach-proof and recyclable and can be used in at lest six runs without loss in catalyst activity.

Solid-State Structure of Binuclear Transition Metal Bisphosphine Complexes. Rotation-rate-dependent MAS Spectra of Dipolar and Scalar Coupled Four-Spin Systems of Dimeric Palladium(I) Complexes

Straightforward 31P MAS investigations were carried out in the solid state on a series of [Pd2X2(dppm)2] (X=Cl, Br, I) complexes in order to study the effects that ring distortion, conformations, and crystal packing have on the solid-state spectra of these molecules and also to get experimental values for certain scalar couplings (e.g. two-bond trans P–Pd–P) normally not available from solution-state spectra. The phosphorus nuclei in these structures form a tightly dipolar coupled four-spin system which cannot be treated by the averaged Hamiltonian theory. The presence of strong homonuclear dipolar couplings, especially under the so called “rotational resonance” conditions, was expected to add to the complexity of the spectra. It turned out that the molecules are asymmetric in solid state but the rotation-rate dependence of the spectra is simpler than expected. Whereas trans spin-pairs with small isotropic chemical shift differences show a J-recouping phenomenon at modest MAS frequencies, the strongly dipolar coupled cis pairs (outside of the real rotational resonance conditions) do not have a characteristic effect in the rotation range studied or the effect is beyond the spectral resolution (<250 Hz). At higher MAS frequencies under the assumption of “inhomogeneous” behaviour reliable values of the trans J-couplings are available from the spectra.

Isolation and characterization of an iodide bridged dimeric palladium complex in carbonylation of methanol

Palladium-catalyzed carbonylation of methanol in presence of iodide promoters was investigated. Iodide bridged palladium dimeric complex, [PPh 3CH 3] 2[Pd 2I 6] was isolated from the carbonylation reaction mixture and characterized using X-ray crystallography. Reaction mechanism was proposed based on IR and UV spectroscopic characterizations of catalytic species involved in the catalytic cycle. The isolated dimeric palladium species, [Pd 2I 6] 2− underwent carbonylation to give monomeric species [PdI 3CO] − at atmospheric pressure of carbon monoxide. It was also observed that PPh 3 plays an important role to avoid catalyst deactivation at higher temperatures. Turnover frequency (TOF) of 1052 h −1 was achieved using Pd(OAc) 2–HI–PPh 3 catalyst system at 175 °C.

Dimeric Palladium Complexes with Bridging Aryl Groups: When are they Stable?

Stable dimeric palladium(II) complexes of general formula [Pd(2)(mu-R)(2)(eta(3)-allyl)(2)] (R=haloaryl, mesityl) have been prepared. Their X-ray crystal structures, determined for some of the complexes, show that the two coordination square planes are usually coplanar. The haloaryl complexes are fluxional in solution, showing exchange between cis and trans isomers (relative to the orientation of the two allyl groups in the dimer) through solvent-assisted associative bridge splitting. A number of other ancillary ligands (O,O, S,S, or C,N donors) failed to stabilize the bridging situation. Also, bridging phenyls were unstable. The reasons for this behavior and the formation of alternative compounds in attempts at synthesizing them are fully analyzed and explained. Stable aryl bridges seem to be favored by a combination of factors: the use of ancillary ligands of small size and lacking electron lone pairs, and the use of aryl ligands reluctant to homo and hetero C--C coupling. These seem to be more important factors in the stabilization of bridging aryl complexes than the strength of the bridges themselves.

Novel Bimetallic Macrocyclic Complexes Assembled from PdII and Flexible Pyridyl Dithioether Bridging Spacers

Two novel dimeric palladium(II) complexes with flexible pyridyl dithioether bridging ligands, [Pd(L1)Cl2]2 (1) and [Pd(L2)Cl2] (2), where L1=1,3-bis(2-pyridylthio)propane and L2=1,3-bis(4-pyridylthio)propane, have been synthesized and structurally characterized by X-ray diffraction analyses. In both complexes, each PdII center is four-coordinated in the trans-form by two chloride anions and two nitrogen donors from two distinct ligands. Complex 1 exhibits a unique 20-membered metallomacrocyclic structure with twisted conformation, while 2 shows a dinuclear 28-membered metallomacrocycle with planar conformation. In addition, there are weak C-H ⃛Cl or C-H ⃛S hydrogen bonding interactions in both complexes, resulting in different 3-D supramolecular architectures.

The reactions of palladium(II), thallium(III) and lead(IV) trifluoroacetates with 3β-acetoxyandrost-5-en-17-one: crystal structure of the first trifluoroacetate bridged 5,6,7-π-allyl steroid palladium dimer

A number of metal trifluoroacetates were reacted with the olefin 3β-acetoxyandrost-5-en-17-one ( 6). Palladium(II) trifluoroacetate afforded bis[μ-trifluoroacetato(α-5,7-η-3β-acetoxyandrostenyl-17-one)palladium(II)] ( 20), a new ring B π-allyl steroid–palladium complex, in quantitative yield. Thallium(III) trifluoroacetate gave 3β-acetoxy-5α-hydroxy-6β-trifluoroacetoxyandrostan-17-one ( 16), 3β-acetoxy-6β-trifluoroacetoxyandrost-4-en-17-one ( 9), 3β-acetoxy-4β-trifluoroacetoxyandrost-5-en-17-one ( 10), and 3β-acetoxy-5α,6β-dihydroxyandrostan-17-one ( 17). Lead(IV) trifluoroacetate yielded 9, 10 and 16. 3β-Acetoxy-5α,6β-bis(trifluoroacetoxy)androstan-17-one ( 15), a new compound, was also formed in this reaction. During the course of the lead(IV) studies the dichlorosteroid 21 and the rearranged allylic oxidation product 24 were formed. Their formation was attributed to the generation of lead(IV) chloride in the reaction. Silver(I) and copper(II) trifluoroacetates proved to be unreactive towards 6.

Diphosphines possessing electronically different donor groups: synthesis and coordination chemistry of the unsymmetrical Di(N-pyrrolyl)phosphino-functionalized dppm analogue Ph2PCH2P(NC4H4)2.

The unsymmetrical diphosphinomethane ligand Ph(2)PCH(2)P(NC(4)H(4))(2) L has been prepared from the reaction of Ph(2)PCH(2)Li with PCl(NC(4)H(4))(2). The diphenylphosphino group can be selectively oxidized with sulfur to give Ph(2)P(S)CH(2)P(NC(4)H(4))(2) 1. The reaction of L with [MCl(2)(cod)] (M = Pd, Pt) gives the chelate complexes [MCl(2)(L-kappa(2)P,P')] (2, M = Pd; 3, M = Pt) in which the M-P bond to the di(N-pyrrolyl)phosphino group is shorter than that to the corresponding diphenylphosphino group. However, the shorter Pd-P bond is cleaved on reaction of 2 with an additional 1 equiv of L to give [PdCl(2)(L-kappa(1)P)(2)] 4. Complex 4 reacts with [PdCl(2)(cod)] to regenerate 2, and with [Pd(2)(dba)(3)].CHCl(3) to give the palladium(I) dimer [Pd(2)Cl(2)(mu-L)(2)] 5, which exists in solution and the solid state as a 1:1 mixture of head-to-head (HH) and head-to-tail (HT) isomers. The palladium(II) dimer [Pd(2)Cl(2)(CH(3))(2)(mu-L)(2)] 6, formed by the reaction of [PdCl(CH(3))(cod)] with L, also exists in solution as a mixture of HH and HT isomers, although in this case the HT isomer prevails at low temperature and crystallizes preferentially. Complex 6 reacts with TlPF(6) to give the A-frame complex [Pd(2)(CH(3))(2)(mu-Cl)(mu-L)(2)]PF(6) 7. The reaction of L with [RuCp*(mu(3)-Cl)](4) leads to the dimer [Ru(2)Cp*(2)(mu-Cl)(2)(mu-L)] 8, for which the enthalpy of reaction has been measured. The reaction of L with [Rh(mu-Cl)(cod)](2) gives a mixture of compounds from which the dimer [Rh(2)(mu-Cl)(cod)(2)(mu-L)]PF(6) 9 can be isolated. The crystal structures of 2.CHCl(3), 3.CH(2)Cl(2), 4, 5.(1)/(4)CH(2)Cl(2), 6, 7.2CH(2)Cl(2), 8, and 9.CH(2)Cl(2) are reported.

Binuclear palladium(I) and platinum(I) dimers stabilized by aromatic ligands: synthesis, structural characterization and reactivity with carbon monoxide

Reaction of PdCl 2 with excess of GaCl 3 in aromatic solvents leads to binuclear compounds of the general formula [Pd 2X 2(arene) 2], where arene=C 6H 6, X −=Ga 2Cl 7 − ( 1); arene=C 7H 8, X −=GaCl 4 − ( 2). The solid-state structures of compounds 1 and 2 have been determined by X-ray crystallography. Two molecules of the arene are bound to the dipalladium unit. The compounds 1 and 2 do not react with triphenyl phosphine. Reaction of carbon monoxide with 1 in benzene solution yields [Pd 2(GaCl 4) 2(C 6H 6) 2] ( 3), for which the crystal structure has also been determined. The compound [Pt 2(GaCl 4) 2(C 10H 10) 2]·2C 6H 6 ( 4), which was obtained by reaction of K 2[PtCl 4] with GaCl 3 and naphthalene in a benzene solution, has a similar structure in the solid state.

Diastereomerism in palladium(II) allyl complexes of P,P-, P,S- and S,S-donor ligands, Ph 2P(E)N(R)P(E′)Ph 2 [R=CHMe 2 or ( S)-*CHMePh; E=E′=lone pair or S]: solution behaviour, X-ray crystal structure and catalytic allylic alkylation reactions

The reactions of achiral homodonor diphosphazane ligands, Ph 2P(E)N(CHMe 2)P(E′)Ph 2 [E=E′=lone pair ( 1) or S ( 2)] with the chloro bridged palladium dimers, [Pd(η 3-1,3-R′-R″C 3H 3)(μ-Cl)] 2 (R′, R″=H, Me or Ph) in the presence of NH 4PF 6 give cationic η 3-allyl palladium complexes, [Pd(η 3-1,3-R′-R″C 3H 3)(L-L′)]. Each of these complexes exists as single species in solution except the 1,3-dimethyl allyl complex, which exists as two isomers ( syn/ syn- and syn/ anti-) in solution. Analogous allyl palladium complexes derived from the chiral homodonor diphosphazane ligand, Ph 2PN(( S)-*CHMePh)PPh 2 ( 3) exist as a single species when the allyl moiety is symmetrical (R′=R″=H or Ph) but a 1:1 mixture of two different face-coordinated diastereomers if the allyl moiety is unsymmetrically substituted (R′≠R″). The 1,3-dimethyl–allyl complex, [Pd(η 3-1,3-Me 2C 3H 3){Ph 2PN(( S)-*CHMePh)PPh 2- k 2 P, P}]PF 6 ( 20) exists as a mixture of two isomers. The major isomer ( 20a) has the syn/ syn-allylic arrangement while the minor isomer ( 20b) has the syn/ anti configuration. These isomers ( 20a and 20b) equilibrate in solution via a syn– anti isomerisation pathway. Achiral and chiral heterodonor monosulphide ligands, Ph 2P(S)N(R)PPh 2 [R=CHMe 2 ( 4), ( S)-*CHMePh( 5)] give rise to allyl palladium complexes which exist as several isomers in solution. The presence of a chiral centre in the ligand 5 enables the observation of two NMR distinguishable face-coordinated diastereomers for its allyl complexes. However, the solid state structure of [Pd(η 3-C 3H 5){Ph 2P(S)N(( S)-*CHMePh)PPh 2- k 2 P, S}]PF 6 ( 27) shows the presence of only one diastereomer. Two-dimensional (2D) phase sensitive 1H 1H NOESY and ROESY measurements indicate that the monosulphide complexes undergo exchange by the opening of the η 3-allyl group selectively at the trans position with respect to the greater π-acceptor phosphorus centre to generate a η 1-bonded intermediate. Preliminary studies on the use of the ligands 1– 5 for catalytic allylic alkylation reactions of an unsymmetrically substituted substrate, ( E)-1-phenyl-2-propenyl-acetate are reported.

Highly Selective and Efficient Catalyst for Carbonylation of Aryl Iodides: Dimeric Palladium Complex Containing Carbon—Palladium Covalent Bond.

AbstractFor Abstract see ChemInform Abstract in Full Text.

New bulky phosphinopyridine ligands. P∼N∼C Tridentates in palladium complexes

The sterically bulky pyridinyl phosphine (P∼N) ligands have been prepared from the phosphinylation of 2,4-di-tert-butyl-6-methylpyridine. Due to the steric hindrance, substitution reaction of these P∼N ligands with (COD)PdMeCl yields the chloro-bridged dipalladium species [(P∼N)2Pd2Me2Cl2], in which the P∼N ligand acts as a monodentate. Treatment of these dimeric palladium compounds with AgBF4 in the presence of acetonitrile gives the corresponding C–H activation metal complex [(P∼N∼C)Pd(CH3CN)]BF4. Both spectral and X-ray single-crystal characterization of these palladium complexes are presented.

New Dinuclear Palladium Phosphane Complexes Stabilised by 8‐Thiotheophylline

AbstractThe new purine derivatives SS‐bis‐8‐(thio)‐theophylline (8‐BTTH2) and 8‐phenylthiotheophylline (8‐PhTTH), have been synthesised and characterised by elemental analysis, mass spectrometry and standard spectroscopic measurements. Reaction of 8‐BTTH2 with cis‐[PdCl2(PPh3)2] leads to the formation of the binuclear palladium complex [Pd(PPh3)(μ‐Cl)(8‐TT)]2 (8‐TTH2 = 8‐thiotheophylline) in which PPh3‐induced cleavage of the S−S bridge takes place. This complex reacts with pyridine to give a different dimeric palladium complex, [Pd(μ‐S,N‐8‐TT)(PPh3)Py]2, in which each of the two 8‐thio‐(theophylline) anions binds two metals through the N7 and S atoms. The process leading to the palladium complex has also been elucidated. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2003)

Unparalleled rates for the activation of aryl chlorides and bromides: coupling with amines and boronic acids in minutes at room temperature.

Unparalleled rates for the activation of aryl chlorides and bromides: coupling with amines and boronic acids in minutes at room temperature.